The present invention relates to a process and a device for automatically detecting faults in fabrics and similar textile sheet-like structures by filtering an image to increase the contrast between the overall texture of the article perceived as normal and the local deviations therefrom perceived as faults.
There are already various existing processes in which the surface of the material is scanned either by means of moving light spots or using sensors arranged in series. The luminosity values produced by the optico-electrical signal converters used in these processes may be prefiltered and are then passed to a threshold value stage, ideally after averaging, because it is found that in particular in the case of fabrics, averaging the luminosity values over several individual threads has the effect of increasing the signal to noise ratio and hence of securing significant benefits for the subsequent signal-processing stage.
In the existing processes, the luminosity values are averaged as a result of the shape and size of the scanned or illuminated area or of the individual sensor. In many cases, two scanning systems are operated in parallel in such a way that one has horizontal averaging characteristics and the other has vertical averaging characteristics, the directions of the averaging characteristics corresponding to the directions of the warp and weft threads in the fabric. There are also existing automatic fabric inspection processes in which the luminosity distribution of the fabric surface is analyzed by means of coherent optical Fourier transformation.